The court of appeal in New Caledonia has upheld the 20-year prison sentence given to Aymerick Vakie for killing the vice-president's chief of staff two years ago.
The badly mutilated body of Jean-Pierre Deteix, who was in his early 70s, was discovered next to a beach and his car was later found about 15 kilometres away in St Louis.
Vakie, who is 23, had claimed he met Mr Deteix in Noumea and they went to the nudist beach where he rejected his sexual advances, killed him and stole his car.
Mr Deteix had arrived in the territory in 1969 and worked at the side of the pro-independence leader Jean-Marie Tjibaou and was a co-founder of Radio Djiido.
He had been the Socialist Party's campaign manager in New Caledonia for Francois Hollande's 2012 presidential election.
The killing was widely condemned, with the then French prime minister Manuel Valls expressing his shock.